I. Field Of The Invention
This invention relates to a process for producing released films comprising layers formed by vapour deposition techniques, to the released films so produced, and to substrates coated with the released films by a transfer process.
II. Description Of The Prior Art
In thin film technology, methods to release films from the substrates on which they have been formed are of widespread interest. These may be used, for example, to obtain free-standing films such as metal foils. Another common application is to realize a given coating on a specific substrate that is incompatible with the desired deposition process for that coating; this is based on a transfer process in which the coating is first deposited onto a compatible substrate, the coated substrate is then bonded to the substrate of interest following which the original substrate is removed, thus releasing the coating from the original substrate and transferring it to the substrate of interest. By incompatible substrate, we mean one that would be adversely affected by the deposition process, or conversely, one that would have an adverse effect on the deposition conditions or equipment. In the case of vapour deposition techniques, an example of the former is a thermally sensitive plastic substrate that would be degraded by heat generated in the deposition process. An example of the latter is a paper substrate that would generate such a water vapour outgassing load that required operating conditions in vacuum deposition processes could not be achieved.
Several such release techniques are well known in the art of thin film deposition using vapour processes such as sputtering, evaporation and chemical vapour deposition. One technique is the use of particular substrate materials or coated substrates to which the film of interest is only weakly adhering and from which it may be peeled; examples of these include polytetrafluoroethylene, polytetrafluoroethylene-coated materials, and highly polished stainless steel. A second technique is the use of a parting layer deposited onto the substrate prior to deposition of the film of interest; the parting layer is dissolved in from the edges of the sample to release the film. Many usable parting layer materials are known including inorganic materials such as sodium chloride and calcium fluoride which are deposited by evaporation and are soluble in water, as well organic films such as cellulose acetate deposited by casting and which is soluble in amyl acetate. These and other parting layers are discussed for example in the review article `The Preparation and Use of Unbacked Metal Films as Filters in the Extreme Ultraviolet` by W.R. Hunter in the monograph `Physics of Thin Films`, Volume 7, Academic Press, 1973, p. 43. A third release technique is simply to deposit the film of interest onto a substrate that can be completely dissolved away. The use of thin plastic sheet for this purpose is disclosed in U.S. Pat. No. 4,434,010 issued on Feb. 28, 1984 to Optical Coating Laboratory Inc.
Each of these techniques is useful in specific applications but all have disadvantages that limit their effectiveness or general applicability. They may themselves involve heat-sensitive or outgassing materials (organic parting layers), have specific crystallinity or surface roughness that may impose undesired growth modes on the film of interest (sodium chloride parting layer), involve solvents that may attack the film of interest or the substrate to which it is transferred, or require excessive dissolution times as in the above cited patent. These constraints are particularly significant in the field of optical coatings where dielectric films on plastic substrates are of interest. These typically involve oxide materials evaporated at high temperature due to their high melting points as well as other materials such as magnesium fluoride that needs to be deposited at a substrate temperature in excess of 200.degree. C. to secure good film properties.
Accordingly there is a need for release technologies to be used with vapour deposition processes, that do not require a dissolution step, and are based on materials that are inert and thermally stable over the range of operating conditions for these processes, and provide a smooth, homogeneous and amorphous surface for deposition.